Colon cancer represents a major cause of cancer morbidity and mortality in the U.S. and the world. Colorectal carcinogenesis originates in the context of dysregulated epithelial cell homeostasis, wherein hyperproliferation, hypodifferentiation and metabolic reprogramming reflect recursive mutually-reinforcing mechanisms contributing to progressive genomic instability. A novel paradigm is emerging in which guanylyl cyclase C (GCC), expressed in brush-border membranes of intestinal epithelial cells, and its paracrine ligands organize and regulate the homeostatic integrity of the crypt-villus axis, forming a hormonal tumor suppressor signaling sequence whose dysfunction defines the initiation of neoplastic transformation and creates a permissive niche for tumor progression. Thus, endogenous GCC ligands guanylin and uroguanylin are invariably lost early during the adenoma-to-carcinoma sequence underlying colorectal carcinogenesis. Moreover, GCC activation with diarrheagenic bacterial heat-stable enterotoxins (STs) suppresses colon cancer cell growth in vitro and in vivo, an effect presumably underlying the inverse relationship between the worldwide incidence of colorectal cancer and infections by ST-producing enterotoxigenic bacteria. Importantly, elimination of GCC expression in mice promotes intestinal tumorigenesis produced by carcinogens or inherited germline mutations. These observations suggest that sporadic colorectal tumorigenesis is a process initiated and promoted by dysregulated GCC signaling. The working hypothesis of this proposal suggests that in colorectal cancer GCC is a novel molecular target to prevent neoplastic transformation and inhibit disease progression by specific ligand therapy. In Aim 1, the ability of GCC signaling induced with ST to inhibit processes underlying the initiation of colorectal tumorigenesis, including formation of preneoplastic and precancerous lesions, will be defined employing azoxymethane-induced carcinogenesis in mice. These studies will demonstrate that activation of GCC signaling prevents colorectal tumor initiation. In Aim 2, the ability of GCC signaling induced with ST to inhibit the growth and progression of colorectal tumors will be defined employing APC?716/+ mice, a genetic model of intestinal tumorigenesis. These studies will demonstrate that activation of GCC signaling reduces the growth and burden of colorectal tumors thereby increasing mouse life span. Taken together, studies proposed will establish the utility of GCC as a therapeutic target for bacterial enterotoxins in colorectal cancer. In that context, the near universal overexpression of GCC by intestinal tumors in patients, reflecting loss of endogenous hormone expression, presents a unique therapeutic opportunity for targeted intervention by oral hormone replacement therapy. Results from this proposal will form the basis for future preclinical and clinical investigations employing targeted GCC activation to prevent and treat colorectal cancer in patients.